1. Field of the Invention
The present invention relates generally to an optical power splitter for use in optical communication systems, and, in particular, to an improved structure of a Y-branched optical waveguide and a multi-stage optical power splitter using the same.
2. Description of the Related Art
Optical communication systems are fast-growing areas in communication networks. The “optical communication system” pertains to any system that uses optical signals to convey information across an optical waveguiding medium, such as an optical fiber. An optical waveguide generally consists of a core section configured to propagate an optical carrier signal within the core, and a cladding section surrounding the entire periphery of the core section. Optical elements employing such an optical waveguide include, i.e., an optical power splitter/coupler for splitting or coupling the optical power of the optical signals, and a wavelength division multiplexer/demultiplexer for multiplexing or demultiplexing multiple channels of the optical signal according to the wavelengths selected. A Y-branched optical waveguide is typically used for splitting optical power, and includes an input waveguide for receiving the optical signal, a tapered waveguide for extending the transfer mode of the input optical signal, and a pair of output waveguides for branching out the optical power of the extended optical signal to provide the branched optical power as an output optical signal.
FIG. 1 is a schematic diagram showing a prior art, Y-branched optical waveguide, which includes a substantially straight input waveguide 110 for receiving the optical signal through a first end section 112; a tapered waveguide 120, the width of which increases along the direction of the propagation of the optical signal for receiving the optical signal through a second end section 122 that is coupled with the input waveguide 110; and, a first and a second output waveguide 130 and 140, respectively, extending from third end sections 132 and 142 outwardly, being symmetrical to each other with respect to a center line 126 of the tapered waveguide 120. The Y-branched optical waveguide may be a planar lightguide circuit (PLC) device formed of multiple layers of a high refractive index of the core section and a low refractive index of the cladding section surrounding the core section on a substrate.
FIG. 2 is a schematic diagram illustrating the waveguiding mode of the optical signal propagating in the Y-branched optical waveguiding medium shown in FIG. 1. As shown in FIG. 2, it can be observed that the split optical signals propagate unstably along the length of the first and second output waveguides 130 and 140, respectively. FIG. 3 is a graphic diagram exhibiting the mode profile of the split optical signals in the third end sections 134 and 144 of the first and second output waveguides 130 and 140. In particular, the first and second mode profiles 150 and 170 of the split optical signals indicate what appears in the third end sections 132 and 142 of the first and second output waveguide 130 and 140, respectively. Note that the respective centering lines 160 and 180 of the first and second mode profiles 150 and 170 deviate by a given distance M1 or M2 from the respective center lines 136 and 146 of the first and second output waveguides 130 and 140, thereby exhibiting the mode misalignment. Here, as the optical signal is perpendicularly incident upon the first end section 112 of the input waveguide 110 while the first and second output waveguides 130 and 140 are arranged symmetrically to each other with respect to the center line 126, the amounts M1 and M2 of the above-mentioned mode misalignment are identical to each other. Therefore, this mode misalignment makes the output characteristic of the Y-branched waveguide unstable. As such, the connection of the Y-branched waveguide to other optical waveguiding elements or a subsequent stage of the Y-branched waveguide will influence the output characteristic of the corresponding Y-branched waveguide disadvantageously.
FIG. 4 shows a schematic diagram of the structure of a two-stage optical power splitter using the prior art Y-branched waveguide. FIGS. 5a and 5b each shows a graphic diagram of the respective mode profiles of the optical signals propagating through the two-stage stage optical power splitter. As shown in FIG. 4, the two-stage optical power splitter includes a first Y-branched waveguide 200 having a first input waveguide 210, a first tapered waveguide 220, and a first and a second output waveguides 230 and 240; a second Y-branched waveguide 250 having a second input waveguide 260, a second tapered waveguide 270, and a third and a fourth output waveguides 280 and 290; and, a third Y-branched waveguide 300 having a third input waveguide 310, a first tapered waveguide 320, and a fifth and a sixth output waveguides 330 and 340.
FIG. 5a shows the mode profile 350 of the optical signal appearing in the first end section 222 of the first tapered waveguide 220, in which the input optical signal is perpendicularly incident upon the first end section 212 of the first input waveguide 210. Thus, the alignment between the center mode of the mode profile 350 and a first center line 226 of the first tapered waveguide 220 is achieved. FIG. 5b shows the mode profiles 360 and 380 of the first-branched optical signals appearing in the first end sections 222 and 322 of the second and third tapered waveguides 270 and 320, respectively. The mode centers 370 and 390 of the mode profiles 360 and 380 are arranged to deviate by a fixed distance M3 or M4 respectively from the first and second center lines 276 and 326, thereby exhibiting the mode misalignment.
As the two-stage optical power splitter has a symmetrical structure with respect to the first center line 226, the mode profiles of the second-branched optical signals appearing in the second end sections 284, 294, 334, and 344 of the third to sixth output waveguides 280, 290, 330, and 340 are formed in symmetry with respect to the first center line 226.
FIG. 6 schematically shows the waveguiding mode of the optical signal propagating through the two-stage optical power splitter. As shown in FIG. 6, it is noted that the first-branched optical signal unstably propagates along the longitudinal direction of the first and second output waveguides 230 and 240, and in a similar way, the second-branched optical signal propagates even more unstably along the longitudinal direction of the third to the sixth output waveguides 280, 290, 330, and 340.
FIG. 7 shows a graphic diagram of the first to the fourth mode profiles 410, 420, 430, and 440 of the second-branched optical signals appearing in the second end sections 284, 294, 334, and 344 of the third to the sixth output waveguides 280, 290, 330, and 340, respectively. Note that the optical intensity in the center of the second and third mode profiles 420 and 430 of the first to the fourth mode profiles 410, 420, 430, and 440 is much higher than that in the center of the first and the fourth mode profiles 410 and 440. That is to say, the input optical signal has been subject to a first modal misalignment passing through the first stage of the two-stage optical power splitter and then a second modal misalignment passing through the second stage of the two-stage optical power splitter. Thus, it is also noted that the first to the fourth mode profiles 410, 420, 430, and 440 represent the result of those two successive modal misalignments overlapped.
As appreciated from the foregoing, the prior art, Y-branched optical waveguide of FIG. 1 has some disadvantages in that it generates undesirably uneven output characteristic as the input optical signal propagating thereof has the modal misalignment in the interim. Furthermore, the multi-stage optical power splitter using the prior art, Y-branched optical waveguide as shown in FIG. 4 may also have the same problem in that it will be undesirably subject to the generation of such uneven output characteristic because the input optical signal passing through the optical power splitter will be effected by the successive modal misalignment caused by a multiplicity of Y-branched optical waveguides.